Enhanced oil recovery processes employ thermal methods to improve recovery of heavy oils from subsurface reservoirs. For example, injection of steam into heavy oil bearing formations heats the oil in the reservoir, which reduces the viscosity of the oil and allows the oil to flow to a collection well. A mixture of the oil and produced water that flows to the collection well is recovered to the surface where the oil is separated from the water.
Different approaches exist for generating the steam. Prior once through steam generators (OTSGs) produce a wet steam by a single pass of water through a boiler isolated from fluid communication with combustion used to heat the boiler. An alternative approach utilizes a direct steam generator (DSG) to produce steam by contacting water with products from oxy-fuel combustion.
Effluent from the DSG thus includes carbon dioxide along with the steam from water vaporization and the combustion to limit water replenishing requirements. While some carbon dioxide injection may enhance hydrocarbon recovery and provide another advantage over the OTSG, excess carbon dioxide may cause an adverse impact on the recovery. The DSG can only provide a narrow range of carbon dioxide concentrations even though less or no carbon dioxide may be more effective.
Therefore, a need exists for systems and methods to generate steam with desired concentrations of carbon dioxide and that are more cost efficient.